[Effect of immobilization on the physical properties of cancellous bone: an experimental study]

40 adult new Zealand rabbits were divided into 6 groups, including a control group, which was free from immobilization. The left hind limb of each animal in the experimental groups was immobilized with the knee in extension by plaster cast for different period of time: 2,4,6,8 and 10 weeks for different group. The cancellous bone of upper tibia of each animal was tested systematically. The bone gravity, pointpressing strength, ash ratio and mineral content were measured and compared group by group to investigate their changes under different conditions. Following immobilization, the quantitative reduction of each parameter varied with the length of immobilization, marked in the first 6 weeks but gradually slowed down. It was most obvious in pointpressing strength, then mineral content, and the least the apparent gravity, denoting that the bone strength was most sensitive to immobilization. The reduction of mineral content exceeded that gravity, indicating the main loss was the former. Loss of point-pressing strength and mineral content was more prominent in the posterior part of the plateau, the weight-bearing area, than at the anterior and intercondylar parts. So was the metaphyseal area as compared with the epiphyseal area. In general, changes in bone gravity was directly in proportion with that of bone strength.